A number of enzymes are secreted in the gastrointestinal tract of animals to digest food. Each of these enzymes acts on specific components in a specific environment of a part of the gastrointestinal tract. Pepsins, for example, are active in the acidic environment of the stomach, whereas other proteases such as chymotrypsin and carboxypeptidases show activity in the upper part of the small intestine at pH 6-7. Many such enzymes need a precursor before being activated. Pepsin, for example, is only formed from pepsinogen in an acidic environment. Chymotrypsin and carboxypeptidases are both secreted in an inactive form and are activated by the protease trypsin.
The digestion of fat is a complex process. Most fat in diets for animals is available in the form of triglycerides. These triglycerides are hardly if at all absorbable by the intestine and need to be degraded to mono- and diglycerides, glycerol and free fatty acids. This conversion is catalyzed by the enzyme lipase which is secreted by the pancreas. This enzyme is active on the interface of water and oil. For good digestion it is essential to have very small droplets of fat in an oil-in-water emulsion. Emulsifiers are surface-active substances that allow the dispersion of fat in a water phase. The most important emulsifier in the gastrointestinal tract is bile. Bile is secreted by the liver and may be stored in the gall bladder. Bile contains a.o. bile acids and salts, cholesterol and phospholipids. Small particles, micelles, are formed by the mixture of the bile components with the (remaining) triglyceride products.
These micelles are diffused to the jejunal epithelial cells, where their contents are released and absorbed. In these epithelial cells, triglycerides are reconstituted. Together with cholesterol, cholesterol esters, phospholipids and proteins, they form new, water soluble, particles termed chylomicrons.
Phospholipids such as lecithin are enzymatically degraded by action of phospholipases A and B which are also secreted by the pancreas.
Lecithin is a mixture of both polar and neutral lipids in which the content of polar lipids is at least 60%. Because of their hydrophobic/hydrophillic character polar lipids (and thus lecithins) are used as emulsifiers. Polar lipids include (glycero)phospholipids and glycolipids. The basic structure of a glycerophospholipid is as follows: ##STR1##
Glycerophospholipids basically consist of a glycerol moiety with fatty acids at the C1- and C2 position. The C3-position is esterified with phosphoric acid. This phosphoric acid is often linked with an alcohol group thus generating the following compounds:
phosphatidyl-ethanolamine (PE; X=ethanolamine) PA0 phosphatidyl-choline (PC; X=choline) PA0 phosphatidyl-serine (PS; X=serine) PA0 phosphatidyl-inositol (PI; X=inositol) PA0 phosphatidyl acid (PA; X=hydrogen) PA0 a) no activity is expected of an added phospholipase in the crop and stomach as a consequence of a mismatch between prevailing pH and pH dependency of the enzyme; and PA0 b) the animals themselves secrete large amounts of this enzyme in the upper part of the small intestine where the prevailing pH is in line with the pH dependency of the enzyme.
Glycerophospholipids bearing just one (instead of the usual two) fatty acid residues are called lyso-phospholipids.
Lecithin is used as an emulsifier in numerous applications including food and feed. Emulsifiers are surface-active substances that allow the dispersion of an oil liquid phase in a water phase. Emulsifiers possess both hydrophillic and lipophilic groups within the same molecule. The ratio of hydrophillic to lipophilic groups, known as the HLB value, is a characteristic indicator for emulsifiers.
Fat-soluble hydrophobic emulsifiers have HLB values in the range of 0 to less than 10 while water-soluble compounds have HLB values between above 10 and 20.
Emulsifiers such as lecithin are added to animal feed to achieve an improved nutritive value of the feed or to achieve a better dispersion in the case of liquid feed. It is also known to add lysolecithin to animal feed (under the trade name Lysoforte.RTM. sold by the Kemin company) which has improved emulsifying properties leading to a better nutritive value (Pluimveehouderij 24: 20-21 (Mar. 18, 1994).
The emulsifying properties of lecithin are not only exploited in livestock production by inclusion of lecithin in dry rations but also in areas where animals are given liquid feed containing a large proportion of fat. These are primarily milk replacements for calves and sow milk substitutes for piglets. The function of lecithins is to produce the finest possible dispersion of the fat in the ready prepared liquid feed. The fine dispersion results in improved digestibility of the fat by the animals. In addition, the lecithin exhibits a favourable effect on the settling of insoluble constituents in a liquid feed.
In recent years, the feed industry has started to use industrially produced enzymes to complement enzymes produced in the gastrointestinal tract of the animals. Examples comprise phytases, .alpha.-amylases, proteases, and various plant cell wall degrading enzymes. However, nowhere in the prior art has the direct addition of phospholipases to animal feed for the purpose of promoting the growth of animals been described since the animals themselves already secrete large amounts of these enzymes in the upper part of the small intestine.
EP-A-0 619 079 discloses the use of inter alia phospholipids as coatings for granulates containing biologically active substances to be include in feed for ruminants. The coating serves to protect the biologically active substances in the rumen, in order to allow subsequent digestion and absorption thereof via the post-abomasum digestive organs. EP-A 0 619 079 further discloses that optionally a phospholipase can also be incorporated in the protective coatings in order to aid in their hydrolysis, however, EP-A 0 619 079 does not disclose or suggest that phospholipases can be added to feed in order to promote growth or to improve the efficiency of feed utilization.
GB-A-2 267 033 is concerned with promoting growth, however, GB-A-2 267 033 teaches to add a kit comprising the phospholipid lecithin together with a Streptomyces strain to silage. It was suggested that the Streptomyces strain is capable of producing a phospholipase A2 during fermentation of the silage. It follows that the use of said kit is limited to animal feed of which the production process comprises a fermentation stage which is compatible with phospholipase production by said Streptomyces strain. Hence, there is still a need for a widely applicable, versatile and ready-for-use phospholipase feed-additive for improvement of the efficiency of feed utilization and/or for the promotion of growth of animals.